Perishable liquids, such as milk and other foods, are typically stored in large tanks. These tanks are provided with heat exchange surfaces in the interior or in the lower portions of the walls of the tanks. The liquid, e.g., milk, is added to the tank and cooled by exposure to the heat exchange to a temperature below that at which significant decomposition and bacterial action takes place. In the case of milk, it is typically stored at a temperature range between slightly above freezing, about 33.degree. F., to below about 40.degree. F. A mean temperature of 37.degree. F. would be typical.
Bulk storage and cooling tanks for milk are found commonly on large dairy farms where the milk is produced. The milk is collected by a bulk truck from the tanks, usually every other day. This means that the tank must be sufficiently large to store and cool 2 days' milk production or the milk produced in four milkings. This creates a number of problems since the milk stored in the tank previously will be at a stable temperature below 40.degree. F. and the milk introduced into the tank in subsequent milkings will be introduced at a temperature near 98.degree. F., the body temperature of the cow. Usually there is some temperature drop in the inlet line so the milk would enter the tank at approximately 90.degree. F. It has been claimed that the quality and shelf life of fluid milk is improved when the incoming milk is cooled before mixing with the milk already in the tank.
In an effort to gain this quality and shelf life improvement, some regulatory agencies have required that some form of precooling equipment be used to cool the milk prior to introducing it into the tank of previously cooled milk. These pre-coolers typically take the form of tubular or plate type heat exchangers, placed upstream of the bulk tank. The precoolers lower the temperature of the milk to below 40.degree.F. prior to introducing it into the tank. Due to the periodic nature of milking, the heat exchange capacity of these devices must be quite high during the short time in which they are used; they remain unused for the major portion of the day. To level out the heat duty on the pre-coolers, they are normally used in conjunction with refrigeration equipment such as water chillers or ice makers which can supply heat exchange capacity by acting as a heat sink during the period of demand upon the equipment. All of this heat exchange equipment, however, takes up considerable valuable space, is costly to purchase and to maintain, is difficult to clean and keep in sanitary condition and may create excess pressure drop and shear which damages the milk.
Applicant has invented a device which eliminates the necessity for pre-coolers stationed in line upstream of bulk milk storage tanks and tanks for storing other perishable liquids. The device reduces the temperature of incoming liquid prior to mixing with the stored mass. The equipment is contained within the tank, easily cleaned, small, inexpensive to purchase and operate, has a very low maintenance, and has very efficient heat transfer. The device rapidly cools the incoming liquid without subjecting it to severe mechanical action which, in the case of milk particularly, may damage it or reduce its quality.
Applicant has devised a method of rapidly cooling perishable liquids on introduction into bulk storage without mixing the incoming warm liquid with the bulk of the cooled liquid in the storage tank. The temperature of the liquid introduced into the tank is rapidly reduced to a stable temperature and the temperature of the mass of liquid stored in the tank never gets above a stable temperature locally and in the mass. In particular for milk, the temperature of milk stored from previous milkings in bulk storage is maintained below 40.degree.F. even when fresh milk is added at each additional milking.
Applicant's device and method use a horizontally disposed spinning disc located at the top of a bulk storage tank to distribute liquid in a film over the surfaces of the tank walls and ends. The heat exchange in the tank is placed in the walls and ends of the tank, not only in the lower portions of the tank, but throughout the walls and ends up to the top of the tank. The liquid is introduced on the rotating disc which propels the liquid in a thin stream off the periphery of the disc onto the upper portions of the tank walls and ends. The liquid flows down the walls and ends in a thin film over the heat exchange surfaces. The thin film provides a very efficient transfer of heat from the liquid into the heat exchange medium and effects an extremely rapid cooling of the liquid as it flows down the walls. The liquid stored in the tank is maintained by the heat exchange in the lower portion of the tank at a stable temperature. As the tank fills up with each addition of new liquid, the cooling of the liquid on the heat exchange surfaces in the upper portions of the tank is sufficient, when combined with the sensible heat of the stored liquid, to insure that the temperature of the mass of liquid stored in the tank never rises above a stable temperature.